A contact image sensor reads an image represented on a document M by irradiating the document M with light from a light source and converting light reflected by the document M into an electrical signal with a light receiving element, and then outputs the result as image data.
The amount of light emitted from the light source can fluctuate depending on various factors. The amount of light from the light source can fluctuate depending on the environmental temperature, for example, due to the temperature characteristic of the light source. The amount of light from the light source can also fluctuate due to chronological factors of the order of a few seconds to a few hours for a shorter elapsed time period to of the order of a few days to a few years for a longer elapsed time period.
For stable reading of the image of the document M by a contact image sensor despite fluctuations in the amount of light from the light source, a bright output that is an output of the contact image sensor when the document M is white is corrected. For example, when the image data output from the light receiving element depending on light reflected by a platen roller described in Patent Literature 1 or a white reference tape described in Patent Literature 2 is given as reference data, the bright output can be corrected by comparing the reference data with predetermined criterion data.